Typical automatic transmissions include a torque converter and a powertrain of a multiple speed gear mechanism connected to the torque converter. In addition, a hydraulic control system is typically provided at an automatic transmission for selectively operating at least one operational element included in the powertrain according to a running state of a vehicle.
A variety of such powertrains and hydraulic control systems have been developed by vehicle manufacturers according to their own schemes. Currently, four-speed automatic transmissions are most often found on the market, but a five-speed automatic transmission is also realized for enhancement of performance of power transmission and for enhanced fuel mileage of a vehicle.
Recently, proceeding from such a five-speed automatic transmission, six-speed automatic transmissions are being researched, such that power transmission performance of an automatic transmission is further enhanced. A six-speed automatic transmission usually includes an additional planetary gearset compared to a conventional five-speed automatic transmission.
Prior art hydraulic control systems of a six-speed automatic transmission for a vehicle use multiple solenoid valves, pressure control valves, and spool valves. This significantly increases the weight and cost of an automatic transmission.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.